The invention relates to methods for illuminating light valves such as those used in color video displays and in particular relates to methods of illuminating such light valves with improved throughput and adjustable color balance.
A need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. A need also exists for a replacement for the conventional cathode-ray tube used in many display devices including computer monitors, conventional and high-definition television receivers and large-screen displays. Both of these needs can be satisfied by display devices that incorporate a light valve that uses as its light control element a spatial light modulator. Spatial light modulators are typically based on liquid crystal material, but may also be based on arrays of moveable mirrors.
Liquid crystal-based spatial light modulators are available in either a transmissive form or in a reflective form. The transmissive spatial light modulator is composed of a layer of a liquid crystal material sandwiched between two transparent electrodes. The liquid crystal material can be either ferroelectric or nematic type. One of the electrodes is segmented into an array of pixel electrodes to define the picture elements (pixels) of the transmissive spatial light modulator. The direction of an electric field applied between each pixel electrode and the other electrode determines whether or not the corresponding pixel of the transmissive spatial light modulator rotates the direction of polarization of light falling on the pixel. The transmissive spatial light modulator is constructed as a half-wave plate and rotates the direction of polarization through 90xc2x0 so that the polarized light transmitted by the pixels of the spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel.
Reflective liquid crystal-based spatial light modulators are similar in construction to transmissive liquid crystal-based spatial light modulators, but use reflective pixel electrodes and have the advantage that they do not require a transparent substrate. Accordingly, reflective spatial light modulators can be built on a silicon substrate that also accommodates the drive circuits that derive the drive signals for the pixel electrodes from the input video signal. A reflective light valve has the advantage that its pixel electrode drive circuits do not partially occlude the light modulated by the pixel. This enables a reflective light valve to have a greater light throughput than a similar-sized transmissive light valve and allows larger and more sophisticated drive circuits to be incorporated.
As with the transmissive spatial light modulators, the direction of an electric field (in this case between the transparent electrode and the reflective electrode) determined whether or not the corresponding pixel of the reflective spatial light modulator rotates through 90xc2x0 the direction of polarization of the light falling on (and reflected by) by the pixel. Thus, the polarized light reflected by the pixels of the reflective spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel.
The resulting optical characteristics of each pixel of both the transmissive and reflective spatial light modulators are binary: each pixel either transmits light (its 1 state) or absorbs light (its 0 state), and therefore appears light or dark, depending on the direction of the electric field.
To produce the grayscale required for conventional display devices, the apparent brightness of each pixel is varied by temporally modulating the light transmitted/reflected by each pixel. The light is modulated by defining a basic time period that will be called the illumination period of the spatial light modulator. The pixel electrode is driven by a drive signal that switches the pixel from its 1 state to its 0 state. The duration of the 1 state relative to the duration of the illumination period determines the apparent brightness of the pixel.
Ferroelectric liquid crystal-based spatial light modulators suffer the disadvantage that, after each time the drive signal has been applied to a pixel electrode to cause the pixel to modulate the light either transmitted/reflected by it, the DC balance of the pixel must be restored. This is typically done by defining a second basic time period called the balance period, equal in duration to the illumination period, and driving the pixel electrode with a complementary drive signal (reverse representation) having 1 state and 0 state durations that are complementary to the 1 state and 0 state durations of the drive signal (positive representation) during the illumination period. The illumination period and the balance period collectively constitute a display period.
To prevent the complementary drive signal from causing the display device to display a substantially uniform, grey image, the light source illuminating the light valve is modulated, either directly or with a shutter, so that the light valve is only illuminated during the illumination period, and is not illuminated during the balance period, as depicted in FIG. 1. However, modulating the light source as just described reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary. This means that a light source of approximately twice the intensity, with a corresponding increase in cost, is necessary to achieve a given display brightness for ferroelectric liquid crystal-based spatial light modulators. Additionally or alternatively, projection optics with a greater aperture, also with a corresponding increase in cost, are necessary to achieve a given brightness.
To produce color output required for conventional display devices, a single spatial light modulator may be used or multiple spatial light modulators may be used. In order to produce a color output from a single spatial light modulator, the spatial light modulator is illuminated sequentially with light of different colors, typically red, blue, and green. This sequential illumination may be accomplished using multiple light sources, each having one of the desired illumination colors, or by using a xe2x80x9cwhitexe2x80x9d light source with sequential color filtering. For purposes of this description a xe2x80x9cwhitexe2x80x9d light source is one that emits light over a broad portion of the visible light spectrum. In either case, each of the sequential colors is modulated individually by the spatial light modulator to produce three sequential single-color images. If the sequence of single-color images occur quickly enough, a viewer of the sequential single-color images will be unable to distinguish the sequential single-color images from a full-color image.
When the single spatial light modulator used to produce color output is a ferroelectric liquid crystal-based spatial light modulator, DC balance must be restored, as previously discussed. Typically, DC balance is restored after each of the sequential colored illuminations as depicted in FIG. 2. Modulating the light source in this manner reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary.
To produce color output using multiple spatial light modulators, each of the spatial light modulators is simultaneously illuminated with a different colored light. This can be accomplished using multiple light sources, each having one of the desired illumination colors, or by using a xe2x80x9cwhitexe2x80x9d light source with a color separator. Typically three spatial light modulators are used, one illuminated with red light, one with blue light, and one with green light. Each of the spatial light modulators modulates the colored light that illuminates it to form a single-colored image, and the single-colored images from each of the spatial light modulators are combined into a single full-color image.
When the three spatial light modulators used to produce color output are ferroelectric liquid crystal-based spatial light modulators, DC balance of each of the spatial light modulators must be restored. Typically, DC balance is restored simultaneously to each of the spatial light modulators (S.L.M.s) after a simultaneous illumination period, as depicted in FIG. 3. Modulating the light source in this manner, once again, reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary.
FIG. 4A shows part of a display device incorporating a conventional transmissive light valve 2 including a single transmissive liquid crystal based spatial light modulator 4. Other principal components of the display device are the polarizer 6, the analyzer 8, and the color sequencer 9. The light valve is illuminated with light from the xe2x80x9cwhitexe2x80x9d light source 10, the efficiency of which may be improved using a reflector 12 and collector optics 14 that concentrate the light towards the polarizer 6. The light output by the light valve passes to the output optics 16 that focus the light to form an image (not shown). The light valve, light source (including reflector and collector optics) and output optics may be incorporated into various types of display device, including miniature, wearable devices, cathode-ray tube replacements, and projection displays.
Light generated by the light source 10 passes through the polarizer 6. The polarizer polarizes the light output from the light source. The polarized light is then transmitted to the color sequencer 9. The color sequencer, allows only a portion of the light in a particular color waveband to pass, filtering the remaining wavelengths of light.
FIG. 4B is a front view of the particular type of color sequencer shown in FIG. 4A. This type of color sequencer 9 is a wheel 18 that can spin around a pivot 20 driven by a stepper motor 22. The wheel includes several filter windows 24 that allow only a particular waveband of light to pass, blocking the remaining light. Blue, Green and Red filer windows are depicted that allow only a blue, a green, or a red waveband of light, respectively, to pass. A color sequence controller 26 is connected to the stepper motor. The controller 26 directs the stepper motor to rotate the wheel around the pivot in the direction indicated by arrow 28, to stop the wheel when the next window 24 is aligned with the spatial light modulator 4, and to begin rotation again after a given period of time has elapsed. Thus, the spatial light modulator is illuminated sequentially with polarized light that is in a blue waveband, a green waveband, and a red waveband.
The spatial light modulator 4 is divided into a two-dimensional array of picture elements (pixels) 30 that define the spatial resolution of the light valve. The direction of an electric field in each pixel of the spatial light modulator 12 determines whether or not the direction of polarization of the light reflected by the pixel is rotated by 90xc2x0 relative to the direction of polarization of the incident light. A substantially reduced number of pixels 30 are shown to simplify the drawing. For example, in a light valve for use in a large-screen computer monitor, the reflective electrode could be divided into a two-dimensional array of 1600xc3x971200 pixel electrodes.
Referring back to FIG. 4A, the light transmitted by each pixel 30 of the spatial light modulator passes to the analyzer 8 and is output from the light valve 2 depending on whether or not its direction of polarization was rotated by the spatial light modulator. The light output from the light valve 2 passes to the output optics 16 to form an image (not shown). This image will consist of green pixels if the color sequencer 9 is in the position shown in FIG. 4B. The following two images output by the light valve 2 will consist of blue pixels and red pixels, respectively. If these images occur quickly enough in sequence, a viewer will see what appears to be a full color image.
FIG. 5 depicts part of a prior art display device incorporating a conventional reflective light valve 39 including a single reflective spatial light modulator 40. It is noted that throughout the following description, elements that are identical to elements previously described are indicated by like reference numerals and will not be described again. The reflective light valve 39 operates in essentially the same manner as the transmissive light valve 2, except that the light transmitted by the color sequencer 9 is reflected by the spatial light modulator 40 rather than being transmitted through it. The reflective spatial light modulator 40 is similar to the previously described transmissive spatial light modulator 4 inasmuch as it is divided into a two-dimensional array of picture elements (pixels) 30 that define the spatial resolution of the light valve 39. In addition, the direction of an electric field in each pixel of the reflective spatial light modulator 40 determines whether or not the direction of polarization of the light reflected by the spatial light modulator 40 at that pixel is rotated by 90xc2x0 relative to the direction of polarization of the incident light.
In the configuration depicted in FIG. 2, the reflective light valve 39 is configured with the light from the light source 10 illuminating the reflective spatial light modulator 40 at an incident angle xcfx86 from the perpendicular. The light reflected from the spatial light modulator is also reflected at an angle xcfx86 from the perpendicular in a direction opposite that of the incident light. Thus, the angle between the light illuminating the spatial light modulator and the light reflected from the spatial light modulator is equal to 2xcfx86. This angle allows the light reflected from the spatial light modulator 40 to transmit unobstructed to the analyzer 8 and allows for a compact overall design.
FIG. 6 depicts part of another prior art display device that, like the device shown in FIG. 5, incorporates a conventional reflective light valve 39 including a single reflective spatial light modulator 40. This display device is distinct from those previously described inasmuch as it utilizes a beam splitter 44. The beam splitter reflects the light from the light source 10 towards the reflective spatial light modulator 40 after it has been polarized by polarizer 6. At the same time, the beam splitter functions to transmit the light reflected from the reflective spatial light modulator towards the analyzer 8. Alternatively, the components could be rearranged (not shown) so that the beam splitter transmits light from the light source towards the reflective spatial light modulator while reflecting the light reflected from the spatial light modulator towards the analyzer.
Using a beam splitter in the manner described offers the advantage that the spatial light modulator 40 can be illuminated from, and reflect light along a path perpendicular to the spatial light modulator. This eliminates any distortion that may result from illuminating the reflective spatial light modulator from an angle xcfx86 as shown in FIG. 5.
FIGS. 7-9 each depict part of a prior art display device incorporating a conventional triple reflective light valve 46 that includes three reflective liquid crystal-based spatial light modulators 40. Each of the triple reflective light valves depicted operates in a similar manner to the display devices previously described. First, the light valve 46 is illuminated with light from the xe2x80x9cwhitexe2x80x9d light source 10, the efficiency of which may be improved using a reflector 12 and collector optics 14 that concentrate the light towards the polarizer 6. The polarized light is then reflected by the beam splitter 44 towards a color separator.
In FIG. 7, the color separator is a series of three dichroic plates 48, 50, and 52, each having an associated reflective spatial light modulator 40. Each of the dichroic plates is configured to reflect light in a band of wavelengths (colorband) particular to that dichroic plate and to pass the remaining wavelengths of light. Thus, a particular portion of the color spectrum from the light generated by the xe2x80x9cwhitexe2x80x9d light source 10 may be reflected by each dichroic plate towards its associated reflective spatial light modulator 40 simultaneously. This eliminates the need for the previously described sequential illumination, and improves the perceived brightness of the color pixels passing through the analyzer 8.
For example, the dichroic plate 48 nearest the beam splitter 44 might reflect red-colored light toward its associated spatial light modulator 40 while the center dichroic plate 50 reflects green-colored light toward its associated spatial light modulator and the remote dichroic plate 52 farthest from the beam splitter reflects blue-colored light towards its spatial light modulator. When the light source 10 is ON, as shown, the colored light reflected by the dichroic plates passes to each of the three reflective spatial light modulators 40. Each of the three reflective spatial light modulators is capable of reflecting pixels of the colored light back at its associated dichroic plate in a manner consistent with the above description of the operation of the reflective spatial light modulator.
The pixellated light reflected by each of the spatial light modulators 40 will consist entirely of wavelengths in the colorband first reflected by the associated dichroic plate. Thus, the vast majority of the pixellated light reflected by each spatial light modulator 40 will be reflected by its associated dichroic plate 48, 50, 52 back toward the beam splitter 44. The beam splitter transmits this pixellated light towards the analyzer 8 and is output from the light valve 46 depending on whether or not its direction of polarization was rotated by the spatial light modulator. The light output from the light valve 46 passes to the output optics 16 to form an image (not shown). This image will be a color image consisting of a combination of the red, blue and green colored pixels from all three spatial light modulators that pass through the analyzer.
In FIG. 8 the color separator is a color separation cube 54, sometimes known as an x-cube or crossed-dichroic cube. As with the three dichroic plates depicted in FIG. 7, the color separation cube separates three distinct colorbands from the xe2x80x9cwhitexe2x80x9d light created by light source 10 and directs each of the colorbands to a particular spatial light modulator 40. The color separation cube 54 also recombines the light reflected from each of the spatial light modulators 40 and directs the combined light toward the analyzer beam splitter 44. The use of a color separation cube allows for a more compact design utilizing three spatial light modulators than can be achieved using three separate dichroic plates 48, 50, 52.
In FIG. 9, the color separator is a three-prism color separator 56 (sometimes known as a Philips cube or Philips prism). The design and use of a three-prism color separator is described in detail in U.S. Pat. No. 5,644,432, the contents of which are incorporated herein by reference. Like the previously described color separators, the three-prism color separator separates three distinct colorbands from the xe2x80x9cwhitexe2x80x9d light created by light source 10 and directs each of the colorbands to a particular spatial light modulator 40. The three-prism color separator 56 also recombines the light reflected from each of the spatial light modulators 40 and directs the combined light toward the beam splitter 44. The three-prism color separator has the advantage over the three dichroic plates 48, 50, 52 and the color separation cube 54 since it typically does a better job of recombining the reflected light from each of the spatial light modulators into a single color image, the color separator consists of three dichroic plates 42, 43, and 44.
In each of the previously described light valves, maintaining an appropriate balance between each of the three color (red, blue and green) pixellated images is critical to the accurate reproduction of colors in the displayed image. The task of maintaining an appropriate color balance can be a difficult problem since many xe2x80x9cwhitexe2x80x9d light sources are inherently unbalanced and the characteristics of the light they produce can change over time. For example, some types of arc-lamp produce far more green light than they do red or blue light at a given xe2x80x9cwhitexe2x80x9d light intensity level. The relative level of green, blue and red light generated by a xe2x80x9cwhitexe2x80x9d light source can also change with operating conditions including items such as operating temperature, operating voltage, age of the light source, contamination, etc.
One technique which has been used to compensate for the unbalanced xe2x80x9cwhitexe2x80x9d light source, is to attenuate the modulation of the spatial light modulator illuminated with the highest intensity color. Thus, in a single spatial light modulator system with high intensity green light relative to the blue and red, the spatial light modulator attenuates the modulation of the green light. This is done by temporally modulating the light transmitted/reflected by each pixel such that the duration of the 0 state relative to the duration of the illumination period is extended to reduce the apparent brightness of the pixel. A similar technique can be used with the green illuminated spatial light modulator in a three spatial light modulator system.
Reducing the intensity of the higher intensity colored light at the light output by attenuating the spatial light modulator has the disadvantage that it reduces the throughput of the light valve and reduces the color resolution of the light valve. For example, if the intensity of the green component of the xe2x80x9cwhitexe2x80x9d light is twice that of the red and blue components, and a spatial light modulator is normally capable of producing 256 grayscale levels during an illumination period, 128 grayscale levels will be used to attenuate the green light. This will effectively reduce to 128 the number of grayscale levels that can be used to display the image.
Consequently, what is needed a method of illuminating a light valve that provides improved light throughput and color balance while maintaining the color resolution capability of the light valve.
The invention provides a method of illuminating a light valve using a light source with modulated intensity to improve light throughput and color balance while minimizing loss of color resolution. The method begins by providing a light valve that includes a light input, a light output and a spatial light modulator. The spatial light modulator includes an array of pixels, each pixel in the array capable of modulating light traveling along an optical path that intersects the pixel between the light input and the light output. Next, the spatial light modulator is illuminated through the light input with light generated by a light source having a nominal lamp power dissipation level. Image data is then provided and the array of pixels is configured based on the image data during a display period so that the image data is represented in light from the light source received at the light output. During a portion of the display period, the intensity of the light generated by the light source is increased to a high level above the nominal lamp power dissipation. During another portion of the display period, the intensity of the light generated by the light source is decreased to a low level below the nominal lamp power dissipation level. The average lamp power dissipation level over the display period is maintained at a level that does not exceed the nominal lamp power dissipation level. Finally, new image data is provided and the process from that point forward repeats.
The spatial light modulator provided may be a ferroelectric liquid-crystal based spatial light modulator, in which case, configuring the array of pixels includes representing the image data in the light from the light source as a positive representation during an illumination portion of the display period and as a reverse representation during a balance portion of the display period. In addition, increasing the intensity of the light occurs during the illumination portion, and decreasing the intensity of the light occurs during the balance portion and may include completely turning off the light source. Further, a brightness user interface may be provided which allows selection of a desired brightness level, and the high level may be set based on the desired brightness level selected.
The light valve provided by the method according to the invention may additionally include a color sequencer for sequentially selecting one of a first, a second, and a third colorband of light that may reach the light output. When the color sequencer is set to the first colorband of light, first colorband image data is provided. The array of pixels is then configured based on the first colorband image data during a first colorband period so that the first colorband image data is represented in the first colorband light received at the light output. Similarly, while the color sequencer is set to the second and then the third colorband of light, second and then third colorband image data is provided, respectively. The array of pixels is configured during a second and third colorband period, respectively, so the second and then the third colorband image data is represented in the second and third colorband light received at the light output, respectively.
During each of the first, second, and third colorband periods, the light generated by the light source is modulated, but the average lamp power dissipation level over the first, second, and third colorband periods is maintained at a level that does not exceed the nominal lamp power dissipation level. Modulating the light source may include turning off the light source during a portion of the first, second, and third colorband period when the spatial light modulator provided is a ferroelectric liquid crystal-based spatial light modulator. Additionally, modulating the light source may include setting the intensity of the light to a first, second, and third level during the first, second, and third colorband period, respectively. The first, second, and third levels may be set to balance to first, second, and third colorband of light at the light output. The first, second, and third levels may be set based on a measured intensity of each of the first, second and third colorband of light. Alternatively, the first, second, and third levels may be set based on desired color balance selected on a color balance user interface.
The light valve provided by the method according to the invention may alternatively include a first, a second, and a third spatial light modulator, each with a first, a second, and a third array of pixels, respectively. Each pixel in each of the first, the second and the third array of pixels is capable of modulating light traveling along an optical path that intersects the pixel between the light input and the light output. In addition the light valve with three spatial light modulators includes a color separator for directing a first, a second, and a third colorband of light from light received at the light input to the first, the second, and the third spatial light modulator, respectively. The light valve may also include a first, second, and third shutter between the color separator and the first, second, and third spatial light modulator, respectively.
The first, the second, and the third array of pixels may be configured to encode a first, a second, and a third representation of the first, the second, and the third colorband image data in the first, the second, and the third colorband of light, respectively, during a display period. The intensity of the light generated by the light source is modulated during the display period while an average lamp power dissipation level that does not exceed the nominal lamp power dissipation level over the display period is maintained. The modulation of the intensity of the light generated by the light source may be used to control a color balance of the first, the second, and the third colorband of light received at the light output. In addition, both the timing of the modulation of the light source and the level of intensity modulation may be adjusted to control color balance.
Each of the provided first, the second, and the third spatial light modulators may be ferroelectric liquid-crystal based spatial light modulators. In such a case, each of the spatial light modulators is independently configured so that the first, the second, and the third array of pixels encode a positive representation of the first, the second, and the third colorband image data, respectively, for approximately half of the display period and encode a reverse representation of the first, the second, and the third colorband image data, respectively, for substantially the remainder of the display period. The positive representations may by configured in all three spatial light modulators simultaneously, one of the spatial light modulators may display the positive representation while the other two display a reverse representation, or all start of the positive representations may be configured in a staggered manner. The first, second, and third shutters are used to prevent the first, second, and third colorband of light from reaching the first, second, and third array of pixels, respectively, when they are configured to encode reverse representations.
Accordingly, the method of illuminating a light valve according to the invention provides improved light throughput and color correction. Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating, by way of example, the principles of the invention.